Harnessing Disorder of Multimode Fibres to Achieve Information Security on the Physical Layer

Digital communication has changed life considerably in a short time and has an impact on all domains . As a result , the risk of novel cyber attacks is increasing rapidly and requires new methods to ensure information security . In particular , optical communication networks have become a crucial environment , because they represent the backbone of the global digital infrastructure . Multimode fibres are a promising link type in this context , as its spatial paths can enhance network capacities significantly . This dissertation investigates an approach to achieve information-theoretic secure data exchange in optical multimode fibres by utilising physical effects in the fibre channel . The objective is for a transmitter ( Alice ) to exploit inherent disorder in multimode fibre to offer a legitimate message receiver ( Bob ) a decisive advantage over an eavesdropper ( Eve ). The clue relies on both modal crosstalk as well as mode-dependent loss that lead to an imbalance among distributed receivers on the fibre channel . Consequently , equalisation between Alice and Bob necessarily does not apply to Eve . This technique is called physical layer security and is experimentally implemented on a multimode fibre for the first time . By measuring the optical transmission matrix , Alice and Bob can characterise their channel for calibration and identify suitable spatial paths . For this purpose , both holographic methods and smart techniques based on neural networks are investigated and compared . By using the tranmsission matrix , Alice and Bob can determine an optical pre-distortion for controlling light propagation through the fibre and exchange information . In turn , Eve must process her channel mathematically . This asymmetry can provide benefit by implementation of sophisticated transmission strategies . Therefore , an attacking experiment was carried out in which Bob and Eve share 50 % each of the transmitted power of a 55-mode fibre . It is shown that through implementation of special channel coding an information-theoretic secure data exchange can be achieved in which 2 bit can be securely transmitted per channel use , although Eve has complete channel state information . Prospectively , the achievable information security could be further enhanced by investigating effects on induced mode mixing or time-varying transmission properties in the fibre channel . D-shaped fibres or external mechanical influences such as bending or twisting can introduce mode mixing or time variance . The results obtained in this dissertation demonstrate the experimental feasibility of physical layer security on a multimode fibre channel for the first time and provide a complement for secure data transmission in optical communication networks of future infrastructures that use spatial information paths.